Mainly in the field of manufacture of photographic papers or coated films, curtain coating methods and apparatus are widely known and used. Typically a continuous web or sheets are continuously moved below a coating hopper. One or more liquid compositions are provided from a hopper arrangement in the form of a liquid curtain.
For the manufacture of photographic papers, liquid compositions are used of relatively low viscosity, generally less than about 150 cP (centipoise), most in the range from about 5 to about 100 cP.
The manufacture of photographic papers is a tremendously difficult art requiring extremely accurate control. The practical use of curtain coating provides a number of difficulties coming with a need for an extremely uniform coating on the one hand and a need for coating of substrates in form of a continuous web at high speeds on the other hand.
A number of problems associated with curtain coating have been addressed in the prior art and many proposals have been made to overcome such problems.
Besides obtaining a free-falling curtain having uniform curtain characteristics over its width perpendicular to the moving direction of the substrate, one of the most often addressed problems for coating at speeds higher than approximately 150 m/min is the displacement or deformation of the curtain by the air which is carried along the uncoated substrate due to friction. That air is carried along with the moving substrate to the coating point which designates the location where the coating liquid first contacts the substrate. In the curtain coating process this location has the form of a line across the substrate and is referred to as the dynamic wetting line. The area near the substrate where the air is in motion due to friction is called the boundary layer.
In the prior art a number of problems are described with respect to the air boundary layer.
One of these problems described for instance in U.S. Pat. No. 6,162,502 A is that air is entrained between the substrate and the liquid film and no coherent coating will be obtained at increased coating speeds.
Even if the air is not entrained between the substrate and the liquid film, the air strikes the curtain in the direction of motion of the substrate with considerable force, especially in the case of high-coating speeds. This leads to disturbances mainly in the area of the dynamic wetting line which cause diffused irregularities in the coated film, as described e.g. in U.S. Pat. No. 6,162,502 A and EP 0 489 978 B1.
Two main effects have been observed in the past in view of the boundary layer hitting a curtain. One is that the air layer hits the contact line between the curtain and the web. As the air needs to reverse its flow direction, the displacement of the wetting line is not uniform over the length over the curtain, and the curtain assumes a wavelike or undulating deformation across the web substrate. As a consequence of the curtain deformation the coated layer gets areas of varying thickness of the coating, which means that the coated layer gets band like thickness deviations along the moving direction of the web.
Another effect is that the curtain is blown up in the direction of the motion of the substrate like a balloon. This results not only in deformation of the wetting line but also results in an irregular coating behavior of the curtain transversely to the coating direction and the momentum of the air or the pressure difference over the curtain may temporarily slit the curtain, thus inducing streaks in the coating.
In a curtain coater arrangement involving an air shield located between a roller for supporting and forwarding the substrate a number of methods are known for mitigating the detrimental effect of boundary layer air. One approach is reported in U.S. Pat. No. 3,508,947 to Hughes where the minimization of entrained air on the moving web is proposed by use of an air shield that has been provided with a vacuum manifold which is positioned adjacent to the web to be coated and connected to a vacuum pump to withdraw air therefrom. In this manner Hughes proposes that the multi-layer, free-falling vertical curtain of coating material is shielded from ambient air currents and the air entrained by the moving web is drawn off before the curtain impinges on the moving web at the wetting line.
U.S. Pat. No. 5,976,630 reports a more recent curtain coating practice employing the air shield mainly for the purpose of drawing off air entrained by the moving web as opposed to shielding the free-falling curtain from ambient air currents. This is because curtain coating operations typically include an enclosure to shield the free-falling liquid curtain from ambient air currents. The enclosure is continuously supplied with a laminar low velocity air flow from the top, while at the same time air is exhausted from both the front and rear of the enclosure. It is known that air shield systems employing a single manifold and a single vacuum source have been operated to exhaust higher air volumes in an attempt to remove additional air from behind the free-falling curtain as well as air entrained on the web.
U.S. Pat. No. 5,224,996 to Ghys et al. is reported to teach an alternative design for a curved air shield arrangement close to a backing roller which supports the moving web at the point of impingement. The design taught for the air shield provides for increased resistance to air flow in the gap between the air shield and the backing roller at the end and side regions thereof as compared to air flow resistance at an intermediate region of the shield. The vacuum device communicates with the gap in the intermediate region to reduce air pressure therein. In such manner, there is an improved removal of boundary layer air at the surface of the moving web prior to the impingement point or wetting line which apparently allows a better coating quality at increased speed of the moving web.
EP 0 489 978 B1 further describes additional arrangements to increase the air resistance by further means like protruding parts, strips or even one or more laminae connected to the air shield and directed towards the web. The laminae are taught to extend over the total width of the air shield or a group of smaller randomly placed laminae. The aim which should be reached by such an arrangement is described to obtain a reduced pressure with a low flow rate of evacuated air. Higher flow rates are reported not to be desirable since they can cause non-uniformities inside the air shield. Such non-uniformities are reported to cause band-like disturbances in the coated material.
EP 0 489 978 B1, the disclosure of which is hereby incorporated by reference, further reports that the pressure difference between the ambient air and the inside of the air shield has to be high enough to evacuate the boundary layer of air adhering to the web, but needs to be limited to avoid an air flow in a direction from the coating curtain towards the air shield, that is against the moving direction of the web. It is reported that an air flow from the coating curtain towards the air shield may cause the entire liquid curtain or at least a part of it to become sucked up into the air shield, therefore destroying the coating procedure, which is to be avoided under any circumstances.
Further, it is described to arrange the outlet end of the air shield at a distance between 5 and 30 mm upstream of the wetting line, because smaller distances involve the risk for a swinging curtain to touch and to soil the air shield, thereby interrupting the coating process, whereas larger distances strongly reduce the effect of the air removal and allow rebuilding of a new boundary layer of entrained air.
U.S. Pat. No. 5,976,630 to Korokeyi et al. proposes use two different intake slots in combination with an air shield which slots are connected to one common or two separate vacuum pumps, wherein one air intake slot is dedicated to removing the entrained boundary air layer of the moving substrate and one is dedicated to the removal of the entrained boundary air layer of the free-falling curtain. Further it is proposed to provide fresh, filtered, optionally heated, laminar, low velocity air flow having a speed of about 10 to about 20 ft/mm (about 5 to about 10 cm/s) which is supplied to the enclosure surrounding the free-falling curtain through an upper perforated wall thereof It is further mentioned that the free-falling curtain is to be supplied with fresh air as spent air as withdrawn from the enclosure surrounding the apparatus through exhaust ports in the enclosure. The exhaust ports are described essentially to remove the supplied air to minimize pressure differential across the free-falling curtain. The teaching of U.S. Pat. No. 5,976,630 is intended to reduce or avoid circulation or vortex pattern of air currents along the curtain which is known to cause disturbances in the curtain which in turn can lead to streaks in the coated product.
U.S. Pat. No. 6,146,690 to Kustermann describes an arrangement for curtain coating for instance of a paper web which should prevent forming of air bubbles by parts of a boundary air layer entrapped between the substrate and the coating applied in an amount making the coated product economically unusable at coating conditions where the web has a width up to 4 m and coating speeds at up to 1000 m/min. To achieve this goal, it is proposed to locate a dynamic air pressure sensor in close proximity to the wetting line where a coating medium contacts the material web surface, and where an increased dynamic pressure relative to the normal air pressure should be observable caused by the boundary air layer entrained to the substrate web. The dynamic pressure signal is compared to a predetermined dynamic pressure value and a suction device to remove air entrained to the substrate web and/or the coating curtain is controlled to maintain a predetermined dynamic pressure value near the wetting line on the substrate.
In a further embodiment of the invention described it is proposed to provide a scraper bar for removal of the air entrained with a moving surface of the substrate located upstream from the wetting line to reduce the mechanical power needed for the suction device, and, further, it is suggested to engage an additional suction device producing a partial vacuum on the side of the substrate web facing away from the coating curtain pulling the substrate web against a support element like a backing roll.
U.S. Pat. No. 6,162,502 to Schweizer et al. proposes to engage a suction channel and an air supply channel within an air shield, both engaging a porous layer towards the substrate web. The air supply channel is arranged between the layer suction channel and the dynamic wetting line and the air supply is proposed to be adjusted in function of the extracted air in such a manner that a parabolic velocity profile develops providing an air velocity equal to zero between the air shield and the substrate with the aim to prevent any air flow in front of the wetting line where the coating curtain strikes the substrate. It is pointed out to be important that the air volume to be extracted is not drained from the space between the air shield and the curtain which needs to be avoided according to the teaching of this patent to prevent from any disturbing air flows in front of the curtain.
U.S. Pat. No. 5,624,715 to Gueggi et al. proposes to extract any air entrained with a moving substrate via a slot at the edge of a blade oriented towards the curtain so that the size of the remaining boundary layer striking the curtain is minimized. Further, an air supply opening is proposed at the underside of a lip of the curtain hopper to provide air to this point at a low speed and downwardly deflected, which low speed air flow is also evacuated by the slot of the blade arranged at the edge of the blade facing towards the curtain. By these measures the formation of rotating air turbulences between the blade and the curtain should be avoided which otherwise may divide into individual unstable cells causing the curtain becoming disturbed and unsteady and, consequently, results in a reduced coating quality.
WO 01/16427 A1 assigned to Valmet Corp. proposes a curtain coater with a conventional doctor arrangement upstream in the travel direction of a web substrate in front of an impingement point of the coating mix curtain on the surface of the web. According to the teaching of this document, besides provision of an usual evacuating means within the doctoring means, it is proposed to increase the momentum of the coating mix curtain by making the height of the falling curtain larger and thereby increasing falling velocity so that the coated liquid becomes more energetic to penetrate through the boundary air layer travelling on the web surface. More particularly it is proposed to provide a gas-injection nozzle downstream from the curtain supplying a significant stream of gas, including air or steam, towards the coating curtain near the wetting line so that the combined momentum of the coating mix curtain and the gas jet becomes sufficiently energetic to force the coating mix to penetrate through the boundary air layer travelling on the web surface and thus, the curtain can unobstructedly adhere to the surface of the web.
Although many approaches have been made in the prior art to overcome the drawbacks and problems coming with the use of a curtain coating process, in particular at high coating speeds, there are still remaining drawbacks effecting the quality and cost effectiveness of curtain coating methods, in particular with respect to curtain coating of paper substrates.